Resistors that are used in integrated circuits to set voltage levels and to determine signal delays are typically formed during the manufacture of integrated circuits. These resistors are typically formed using wells, diffusions within wells, or formed using doped polysilicon. Doped polysilicon resistors are preferred because unlike well resistors and diffusion resistors which have significant diode capacitance the doped polysilicon resistors may be formed on thick isolation oxide with very low capacitance.
To reduce cost, resistors are usually formed using masking steps and implantation steps that are already in the integrated circuit manufacturing flow. For example an nwell resistor may be formed using the nwell patterning and nwell implantation steps also used to form p-type metal-oxide-semiconductor (PMOS) transistors. A diffused n-type resistor may be formed using the n-type source and drain patterning and n-type source and drain implantation steps also used to form n-type metal-oxide-semiconductor (NMOS) transistors. A polysilicon resistor may be formed at the same time as the polysilicon transistor gates and may use the polysilicon gate doping or the source and drain doping.
By using existing patterning steps and implantation steps additional cost to form resistors may be avoided.
Since the resistance of the resistor material is determined by the doping concentration, the resistance of the resistor is determined by the length of the resistor body. If high resistance is needed, a very long resistor body may be required. Large area increases cost. To reduce the resistor area and resistor cost lower resistor doping would be required, but this would add an additional resistor doping patterning and implantation steps which also adds cost.